This invention relates to surface acoustic wave resonator devices having grating reflectors, and more particularly to resonator devices used as bandpass filters, oscillator frequency control elements and the like.
There is frequently a requirement in communications and other electronic fields for band pass filters having a narrow frequency range or oscillator frequency control elements. Surface acoustic wave resonator devices may fulfill such a requirement and various surface acoustic wave resonators have been developed.
The surface acoustic wave resonator device includes basically a piezoelectric substrate, at least one interdigital transducer disposed on the piezoelectric substrate for converting an input electric signal to surface acoustic waves propagating on the surface of the piezoelectric substrate, and grating reflectors for reflecting the surface acoustic waves. This kind of surface acoustic wave resonator device is described, for example, in U.S. Pat. Nos. 3,886,504 and 4,166,258.
The impedance of such resonator device must be designed in accordance with the impedance of an external circuit connected thereto. Therefore, a resonator device having high impedance is frequently required as well as a resonator device having low impedance. The optimal impedance of a surface acoustic wave resonator is inversely related to the aperture length of the interdigital transducer as described in a paper entitled "SURFACE ACOUSTIC WAVE RESONATOR DEVELOPMENT" by J. S. Schoenwald et al. presented at Proc. of the 29th Annual Symp. on Frequency Control, page 150-157, May 1975. Accordingly, a surface acoustic wave resonator device having high impedance may be obtained by decreasing the aperture length of the interdigital transducer. However, decreasing the aperture length of the interdigital transducer results in reducing the quality factor Q of the resonator device because a beam of the surface acoustic waves propagating on the piezoelectric substrate expands out of the resonance region by diffraction and energy loss increases. Moreover, it is difficult to manufacture such interdigital transducers having a small aperture length.